Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. Transmission speed ratio is the ratio of input shaft speed to output shaft speed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
When driving conditions change, an automatic transmission changes from one speed ratio to another speed ratio. For example, when a vehicle is cruising using a low speed ratio and a driver demands an increase in wheel torque, the transmission must downshift into a higher speed ratio. For sudden changes in driver demanded wheel torque, the transmission may skip over one or more available gear ratios in a single shift event. Many automatic transmissions have multiple shift elements, such as clutches or brakes, and select particular speed ratios by engaging particular subsets of the shift elements. To perform a shift from one speed ratio to another, one or more previously engaged shift elements are released and one or more previously disengaged shift elements are engaged. Passenger comfort is maximized if the transition is accomplished smoothly. Performance is maximized if the transition is accomplished quickly. These considerations are often in conflict.